In-circuit tests (ICTs) are the tests for electrical properties of electrical elements in printed circuit boards (PCBs) and electrical connections between the electrical elements. The in-circuit tests can be used to detect the potential manufacturing defects in the printed circuit boards, e.g., open or short circuits, and the reliabilities of the electrical elements by measuring the resistances, capacities and other characteristics of each electrical element. The in-circuit tests are standard tests aimed at printed circuit boards. The in-circuit tests offer advantages, such as simple operation, high efficiency and accurate defect positioning, etc.
One approach of in-circuit tests is to produce a via in a multilayer printed circuit board for as a conducting channel between an electrical element and the circuits on the PCB and a test probe. And a test pad coupled with the via is produced on the upper surface of the printed circuit board so that the electrical element and the circuit are electrically connected to the exterior of the printed circuit board. A test probe of a test apparatus contacts with the center of the test pad to test the electrical properties of the printed circuit board.
As shown in FIG. 2, in producing the in-circuit test structure, a via 210 is initially formed in a printed circuit board 200, and then a test pad 230 is formed on the upper surface of the printed circuit 200. Conventionally, the center of the test pad 230 is located right above the via 210, and the center part of the test pad 230 may recess toward the via 210. The recession can cause reduced thickness and the increased resistance of the center part of the test pad 230. When the electrical test is carried out, it often deviates the test result, if the test probe contacts with this part of the test pad 230.
Moreover, typically a via needs to be plugged with soldermask to avoid electrical short and oxidation. For solder-opened via, one side of the via is soldermask plugged, and the other side having a big test pad remains solder-open for contacting the probe tip. During this type of process, the solder mask may seep into the solder-opened side of the via from the soldermask-plugged side, thereby affecting the contact between the test probe and the test pad.
But in the practical testing process, the test data often deviates from the actual data. Sometimes, the test data cannot even be obtained.